Conventionally, a display device which has a resolution of approximately 75 dpi (Dot Per Inch) to 100 dpi is commonly used for a personal computer (hereinafter, referred to as a PC). In contrast, as for a resolution of an image forming apparatus, an electrophotographic printer has a resolution of approximately 600 dpi to 2400 dpi, and an inkjet printer has a resolution of approximately 360 dpi to 2880 dpi. This shows that an image forming apparatus has a higher resolution than a PC display device.
As for gradation, a PC display device provides 256-level gray scale for each of RGB. In contrast, an image forming apparatus providing two-level gray scale is commonly used, and an image forming apparatus provides approximately 64-level gray scale at most. Even an image forming apparatus providing approximately 64-level gray scale often fails to appropriately express gradation in a low-density part and a high-density part of a printed image.
As described earlier, a PC display device which displays a digital image and an image forming apparatus which prints the digital image greatly differ mainly in resolution and gradation.
Therefore, in order to absorb a difference in gradation, an image forming apparatus carries out a halftone process (also referred to as a gradation reproduction process or quantization means) such as a dither method or an error diffusion method in printing a digital image. Such a halftone process has a characteristic of obtaining gradation in exchange for resolution or obtaining resolution in exchange for gradation. The image forming apparatus properly uses a plurality of halftone processes in accordance with a type of a digital image to be printed.
For example, in order to print a text, the image forming apparatus carries out a halftone process which places greater importance on reproduction of resolution than on reproduction of gradation. In contrast, in order to print a photograph, the image forming apparatus carries out a halftone process which places greater importance on reproduction of gradation than on reproduction of resolution. In this case, it is known that a shape (a curved line) of a density characteristic to be reproduced varies depending on a kind and/or a setting of a halftone process.
FIG. 10 is a graph illustrating density characteristics of respective halftone processes. The density characteristics of the respective halftone processes differ among the halftone processes. For example, a halftone process for providing a high resolution such as an error diffusion method or a high lpi (line per inch) dither method has a density characteristic which is close to a density characteristic of an image forming apparatus itself. In contrast, a halftone process for providing a low resolution such as a low lpi dither method tends to have a density characteristic which is close to a straight line (an ideal density characteristic). Note that a curved line which shows such a density characteristic of a halftone process changes over time depending on, for example, conditions such as temperature and humidity, and a usage state.
Therefore, an image forming apparatus having a function of carrying out a plurality of halftone processes carries out process control so as to keep a printing density and a color tone constant among the plurality of halftone processes. Then, the image forming apparatus carries out a density correction with respect to each of the plurality of halftone processes. The process control, which is an image quality adjustment for achieving both user convenience and a stable image quality, is carried out at turn-on, when a change over time or an environmental change is detected, or at a timing at which the number of sheets printed reaches a given number.
Patent Literature 1 discloses a density correction method. According to the density correction method, an arrangement having a function of carrying out plurality of halftone processes carries out a density correction by forming toner patches for all halftone processes that are required to be subjected to the density correction and actually measuring patch densities of the toner patches.
However, according to the density correction method of Patent Literature 1, actual patch density measurement is carried out with respect to each of the halftone processes that is required to be subjected to the density correction. Therefore, this causes problems of (i) costs for materials to be consumed (e.g., toner and ink) and (ii) longer time required for the density correction.
Patent Literature 2 proposes a density correction method such that one of a plurality of halftone processes is set as a reference halftone process, a density correction is carried out with respect to only the reference halftone process by actual patch density measurement, and the density correction is carried out with respect to the other halftone processes by conversion from an output correction value of the reference halftone process (a result of the density correction carried out with respect to the reference halftone process).
Patent Literature 2 can solve the problems of Patent Literature 1. This is because according to Patent Literature 2, an output correction value is found by carrying out actual patch density measurement with respect to only the reference halftone process, and merely conversion from the output correction value of the reference halftone process is carried out with respect to the other halftone processes.